Bispecific ADCs, Strategies to Improve the Therapeutic Window
As we mentioned among preclinical challenges, it is mostly essential for an ADC to be delivered into lysosomes to release the cytotoxic payload. This is vital for the only approved ADC to treat a solid tumor, T-DM1 (Kadcyla), since it carries a non-cleavable linker. However, HER2 receptor, the target antigen for T-DM1, despite its selective expression on breast tumor cells is found to recycle back with high rate after internalization. Overall, this leads to only a small proportion of T-DM1 to reach lysosomes. This phenomenon may stand behind the limited efficacy of T-DM1 that involve its failure to show a benefit in a gastric cancer trial, whereas trastuzumab (unarmed antibody) passed approval. Also, in Phase II and III clinical trials for breast cancer it failed to show higher clinical benefit than standard therapies.
Its benefit seems to be restricted to breast cancer patients with high HER2 expression. Bispecific antibody technology delivers promising results in the way to solve this problem. These antibodies have two paratopes or they can recognize two different antigens, while conventional antibodies are “monospecific” (although bivalent). In the context of enhancing lysosomal trafficking, two particular bispecific ADC - based strategies stands out.
The first bispecific antibody has each paratope recognizing different epitopes on the same antigen. Such bispecific ADCs have been shown to induce receptor clustering on the target cells and increased internalization, lysosomal delivery and on-target cytotoxicity. ZW49 developed by Zymeworks is a bispecific ADC that recognizes two different epitopes on HER2. It is currently in Phase I for the treatment of biliary tract cancer.
Another approach, named as “drag and degrade”, is based on getting help from a receptor that regularly and efficiently traffics to lysosomes. Here, one of the paratopes of the antibody recognizes a “fast-internalizing” receptor, the other recognizes a tumor-specific antigen. HER2/CD63 and HER2/Prolactin receptor bispecific ADCs have been shown to enhance lysosomal delivery and on-target cytotoxicity when compared to HER2 monospecific ADC.
Both targets of bispecific ADC may not have similar contributions. Various parameters such as the affinity of the individual arms, the density of the target, the overall avidity and the valency of the bispecific format determines tumor selectivity. A study testing EGFR/cMet ADC showed that having low affinity EGFR paratope allowed to show toxicity against tumor cells while sparing normal keratinocytes which have moderate level EGFR expression.
Antibodies have relatively larger sizes that hinder the efficient delivery of ADCs to sequestered targets such as the interior of the solid tumors. The importance of this phenomenon is best reminded by the fact that most of the marketed ADCs are approved for the treatment of hematological cancers. Therefore, in recent years, attention is directed towards the use of smaller formats than antibody as a targeting unit of the conjugate. These include antibody fragments, peptides, scaffolds (e.g. centyrins), and even small molecules. Some of these are discussed in the next chapter with clinical applications.
Certain features of smaller format conjugates can potentially improve the therapeutic window. First, they have higher tumor penetration than ADCs due to their smaller physical radius and hence faster diffusion and extravasation coefficients. Second, they have a high plasma clearance rate that may reduce off-target toxicities. However, this may be a disadvantage as well since their concentration in tumors also decreases rapidly. That could be improved via repeated injections or further engineering to enhance their circulation time.
Smaller formats could especially make a difference by increasing the tolerated concentration via minimizing adverse effects. The lack of the Fc domain enables one to avoid cross-reactivity with Fc-receptors in healthy cells and this could improve the tolerability of higher doses. Fc domain of ADCs causes thrombocytopenia and neutropenia via binding to Fc receptors on non-target cells.
Comer F., Gao C., Coats S. (2018) Bispecific and Biparatopic Antibody Drug Conjugates. In: Damelin M. (eds) Innovations for Next-Generation Antibody-Drug Conjugates. Cancer Drug Discovery and Development. Humana Press, Cham. Maruani A. (2018) Bispecifics and antibody–drug conjugates: A positive synergy. Drug Discovery Today: Technologies, 30, 55-61. https://www.zymeworks.com/pipeline
Deonarain, M.P.; Yahioglu, G.; Stamati, I.; Pomowski, A.; Clarke, J.; Edwards, B.M.; Diez-Posada, S.; Stewart, A.C. Small-Format Drug Conjugates: A Viable Alternative to ADCs for Solid Tumours? Antibodies 2018, 7, 16. He, R.; Finan, B.; Mayer, J.P.; DiMarchi, R.D. Peptide Conjugates with Small Molecules Designed to Enhance Efficacy and Safety. Molecules 2019, 24, 1855.
